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The U.S. Senate recently passed an immigration reform bill that would allow many of these people to become citizens. It would also toughen border security, making it more difficult for others to make their way into the country. As Congress reconvenes after the July 4 recess, the debate will now move to the House of Representatives, where the outcome is uncertain.

Crossing from Mexico to the U.S. on foot is already a formidable undertaking. In the state of Arizona, one of the busiest points of entry into the U.S., authorities apprehended some 220,000 people in the last year alone. The previous year they found the remains of 179 people who perished in the brutal Sonoran Desert, where daytime temperatures in the summer often exceed 115°F (46°C).

It's the poor and the desperate from all over Latin America who undertake this journey—men, women, and children willing to take monumental risks for a fresh start. Similar stories have played out many times in U.S. history, with each new wave of immigrants taking a leap of faith for a chance at the land of opportunity.

"We know that in a hundred years the current migrations will become romanticized and mythologized, much like the Irish experience," says University of Michigan anthropologist Jason de León. "If we don't record it now, we're going to lose the story of what really happened in the desert."

Since 2009, de León has been recording the gritty details of the story through his Undocumented Migration Project. He and his team of students interview people in Mexico who are about to attempt the journey, and those who have successfully entered the U.S. through Arizona. They also trek into the desert themselves to document what people leave behind—discarded water bottles, food wrappers, torn photos, even their own bodies.

So far, de León's team has collected 10,000 artifacts that testify to the sometimes tragic journeys. They also photograph objects just as they're found in the desert.

The backpack pictured above displays colors that a young girl might have picked out. Did she get tired and drop this? Was she forced to put it down when her group was apprehended? Did she die in the heat like so many others? Or was this something that a father carried as a reminder of home and family on his long trek? Each scenario is possible.

Many people view such dirty, torn, faded things as trash. But for de León, each is an artifact that tells a story.

—A. R. Williams

Photograph by Michael Wells

One-Stop Shopping

In the town of Altar, Mexico, vendors sell a variety of gear to migrants setting out for the U.S., from backpacks and hoodies to caps and hiking boots. "They're making quite a bit of profit off fear and trying to convince migrants that the newest thing is the best thing," says 36-year-old de León, a National Geographic Emerging Explorer.

"This material evolves pretty quickly," he says. "We're even able to date migrant sites in the desert based on the type of camouflage of the backpack."

Located about 60 miles south of the border, Altar is the last stop in Mexico for migrants who may have come from as far away as Ecuador. Many spend several days here buying supplies and resting up for the journey to come. They then pay a smuggler to drive them north in a van and guide them on a hike into the U.S. that can cover 50 miles or more.

But a growing number now arrive in this area with so little money that they can't afford to hire help, and try to cross the border on their own.

"I met a guy with one arm the other night from Chiapas, Mexico," said de León in June, after returning from a two-week field trip for research. "He had spent five days lost in the desert wandering around without a guide. He didn't even have a backpack, so everything he had on him was in his pockets."

Photograph by Michael Wells

Emergency Rations

Like any good business, a grocery store in Altar, Mexico, stocks its shelves according to what sells best. This aisle isn't meant to appeal to the 10,000 local residents as much as to the migrants who buy fruit-flavored rehydration drinks in bulk. "Out in the desert," says de León, "you find millions of empty bottles."

Photograph by Michael Wells

Personal Belongings

Well-worn socks, an empty medicine pack, a jacket to fend off the chill of nighttime—these are the practical things that de León normally finds in the desert. Some items are meant to be thrown away or left behind. Others, like driver's licenses and personal photographs, may be left by accident or abandoned during a border patrol raid.

Occasionally, migrants decide to take along something incongruous that they end up having to ditch—a baby stroller, a set of curlers, a briefcase, a wheeled suitcase better suited for the airport.

And every once in a while, something from a smuggler turns up.

In one place, de León found a notebook written in code. It was a checklist of all the things a smuggler would need to do in order to get people across the desert—change the van's license plates, for instance, and take a head count. Smugglers often call their customers pollos, or chickens, so people's oval heads, in this case, were listed as huevos, or eggs.

Photograph by Michael Wells

Final Destination

One word encompasses the hopes and dreams of the migrants who make this trek. Someone was likely resting here, in a thicket of bushes and shrubs that hid the spot from view, when he or she carved the journey's destination into this tree trunk with a knife.

"We find lots of graffiti," says de León. "People leave all sorts of traces of their migration—sometimes in purposeful ways like this, and sometimes without thinking about it by throwing down a bag or a water bottle."

Photograph by Michael Wells

Way Station

In this sheltered spot with patches of shade, migrants stop to rest, eat, change clothes, and clean up as best they can for the next leg of their trip. By now they've been walking for several days, through a rugged landscape of cactus and mesquite, and are north of the town of Arivaca, Arizona—past vehicle checkpoints. Smugglers' cars pick them up here and may drive them to Tucson, about 50 miles to the northeast, or to Phoenix, about 140 miles to the northwest. There's no room in a crowded car for backpacks or spare clothes, though, so those things are shed like old skins before a new phase of life begins.

Photograph by Michael Wells

Odd Gadget

A pair of rusted compasses were probably part of someone's school supplies forgotten in the bottom of a backpack. Neither migrants nor smugglers are in the habit of carrying anything that could help them figure out their location. If caught, such items could earn them serious jail time.

Instead, smugglers memorize landmarks and use stars to help them navigate. Migrants are completely at the mercy of the smugglers. "It's easy to get lost out there even with a GPS unit," says de León. "I can't imagine not having anything at all."

Photograph by Michael Wells

Hallowed Ground

Deep in a canyon near Tucson, Arizona, where the terrain is rocky and steep, migrants have created a shrine. They leave rosaries, prayer cards, crucifixes, and candles as signs of their faith and as sources of comfort for those who will pause here in the future.

"I think these shrines are some of the few places that give them a little bit of hope," says de León. "They don't have many resources besides their religious beliefs to get them through this very difficult process."

Photograph by Michael Wells

Name Unknown

The mortal remains of one migrant, discovered in the desert, have been brought to the Pima County Medical Examiner's office in Tucson, Arizona. There were no accompanying documents like a driver's license, but a forensic examination identified this as a male.

Several things could have happened to the IDs that this person would have carried at home: Bandits may have attacked him in the desert and stolen everything he was carrying, including his wallet.

Dehydration and heat stroke may have killed him, and scavenging animals scattered his personal effects across the unforgiving landscape.

Or he may not have been carrying an ID at all, hoping to disguise exactly where he came from. Why? If a migrant from, say, Central America is apprehended in the U.S. and can pass for a Mexican, he'll be deported into Mexico, as opposed to Guatemala or Nicaragua. That will save him many hundreds of miles when he starts north again on a return trip.

Photograph by Michael Wells

Prized Possessions

Registered and bagged, a deceased migrant's personal effects are ready to be put into a storage locker at the medical examiner's office in Tucson.

If the identity of the migrant can't be confirmed in a year, these things will be destroyed. "A lot of people just disappear," says de León. "The system processes them pretty quickly, and it's as if they were never there."

Photograph by Michael Wells

International Pastime

A cap lying in the desert suggests that the owner was a baseball fan. The New York Yankees are popular in Mexico, so a first-time migrant could have bought this in his hometown before heading north.

But someone may just as likely have been to the U.S. already and gotten it there before being deported. De León sees such cases all the time—migrants on their second or third trip north, who have lived and worked in the U.S. and have embraced its culture.

"I ran into a guy in the desert the other day," says de León. "He'd been in the States for some 20 years, spoke perfect English, and been arrested at work. He was hiking in Converse sneakers. He said, 'Look, I'm a hard-working person. I'm a roofer—my pants still have glue on them from the job I was doing that day.'

"People like him are American in so many ways," de León says, "except for the fact that they don't have the paperwork."